Some urea derivatives having nitroso and chloroethyl groups attached to the one or both of the nitrogen atoms of the urea-molecule have been suggested and also sometimes found useful in the treatment of various tumours, either alone or in combination with other cytostatic drugs. Recently such derivatives known from U.S. Pat. No. 4,301,287 have been suggested. Such nitroso- and chloroethyl-derivatives have, however, various serious side effects, and some are rather toxic compounds, and the therapeutic index, the ratio between the toxic dose and the therapeutic dose, is not very favorable.